The present invention relates to a variable modulation communication method and apparatus such a digital radio communication system accommodating a plurality of terminals.
In the conventional radio communication systems, a modulation scheme is fixed for each system, and cannot be changed regardless of the state of a transmission path. For example, digital automobile telephone units in Japan and the Unites States employ a quaternary modulation scheme called .pi./4 shift QPSK (Quadrature Phase Shift Keying), which is invariably used regardless of the conditions. In order to improve the frequency utilization efficiency, a multilevel modulation scheme such as a 16QAM (Quadrature Amplitude Modulation) scheme may be used. If, however, such a scheme is used, the system becomes susceptible to noise, and hence the service area is narrowed.
A method of ensuring a service area by switching modulation schemes depending on the state of a transmission path has already been employed by a telephone modem used for a facsimile apparatus or the like. According to the method, when a transmission path is in a normal state, data is transmitted at 9,600 bps by 16QAM modulation. If a transmission path is in a bad state, however, the number of multivalue levels in modulation is sequentially decreased to 4 and then to 2 in order to maintain the communication quality, thereby transmitting data at only 4,800 bps or 2,400 bps respectively. As described above, the method of variably setting modulation schemes has already been put into practice in communication systems which allow controlled variation in transmission rate of information between users, such as a facsimile system and or a data communication system. However, the method has not yet been employed in communication systems which demand a constant information transmission rate, such as a telephone system, because conventional methods of switching modulation schemes causes a change in transmission rate.
In the conventional communication method, since a constant modulation scheme is used regardless of reception power, if the signal power is strong, the frequency utilization efficiency cannot be improved.
In addition, if modulation schemes are switched when a transmission path is in a bad state, the transmission rate is inevitably decreased. In order to prevent this, a method of increasing the symbol rate in transmission may be employed. For example, the amount of information which can be transmitted can be substantially kept constant by doubling the symbol rate instead of decreasing the number of multivalue levels in modulation from 16 to 4. In this method, however, the filter system and other equipment must be replaced, resulting in difficulty in configuring the hardware. In addition, in radio communication, changes to the occupied bandwidth cause interference with other systems. Therefore a problem is posed in terms of radio regulation.